Butterfly - Ant liason

Ants are usually the number one enemy of butterflies, but in some species, ants tenderly care for butterfly larvae as if they were their own, even raising them in their own nests. But what do the ants get out of it? Abbie Thomas investigates.

It's a sight that never fails to wrench the heart: a helpless butterfly, downed by wind or injury, flapping feebly as it is swarmed over by ants and slowly eaten to death. But some butterflies, instead of succumbing to the enemy ant, have evolved a bizarre but lucrative relationship with this ancient foe.

Members of the butterfly family Lycaenidae have made a pact with the ants: a perilous agreement held together by a complex system of physical features, hormones and behaviour which is so delicate, it only requires one failed link in the system for instant death to occur. Yet the relationship between some ants and butterflies has become so close over millions of years of evolution, that one could not survive without the other.

Just why these two rivals are attracted to each-other and how they manage to get along has been the subject of much study. Now the story has emerged of one of the most elegant and mystifying relationships in the animal world.

Discovery

Right from an early age, butterfly caterpillars are attended by ants. Image: Simon Nally [more pics]

Last spring, a seven year old girl walking around her parent's new property south of Oberon in NSW made an amazing discovery. The farm is one of the few left in the district which had not been extensively cleared, and this was one reason the nature-loving family were attracted to it. They had already contacted NSW National Parks and Wildlife to find out about some frogs they had heard in the area. But when they mentioned where their farm was and described the type of vegetation growing there, National Parks officer Simon Nally became very excited. He realised the farm could well be one of the last refuges for a very rare butterfly - the Bathurst Copper Paralucia spinifera.

The family were sent a brochure on how to identify the butterfly, and one day, their seven year old daughter set out to see if she could find any. She studied the information carefully, and eventually found the food plant of the Bathurst Copper larvae - a bushy plant called Blackthorn which grows in disturbed areas. Then she found more tell-tale signs: leaves chewed off the bottom of the bush, and several adult butterflies flying around. Eventually she spotted the larvae themselves, feeding on the plant and surrounded by attendant ants. This young girl had found a whole new population of Bathurst Copper butterflies, 50km away from the insect's nearest known habitat.

This story illustrates one of the most typical traits of butterflies which live in association with ants - they often live in tiny, isolated patches, far apart from each other, making them vulnerable to local land clearing. But in fact, lycaenid butterflies - of which about half have an association with ants, are widespread across Australia - you just need to know, where, and how to look.

One of the best ways to spot the Bathurst Copper, for example, is to look for leaves chewed off near the bottom of a plant. The larvae of the butterfly actually live in ant nests at the base of the tree. At night, ants shepherd the larvae up the trunk to the nearest leaves, where the larvae can feed all night. So zealous are the ants in their duty, says Simon Nally, that if someone shakes a branch, half the ants begin attacking the shaker, while the rest turn the caterpillar around and start encourging it back to their nest.

Sweet liaison

Professor Naomi Pierce found that caterpillars raised with ants have a much better survival rate than those raised on their own. Image: Harvard news [more pics]

The lycaenid group of butterflies, to which the Bathurst Copper belongs, typically have iridescent blue, purple or copper on the upper wing surfaces. Some species are so vivid they have been seen fluttering above the trees by people flying in planes. But what makes them really special is the amazing relationship that the butterfly larvae, or caterpillars, have evolved with ants.

While the ants look after the caterpillars, payback time comes soon enough. The larvae secrete a sweet liquid from glands on their skin, much like nectar from flowers, and this liquid is irresistible to ants. They lap it up, straight from the surface of the caterpillar. In return, the ants tend the caterpillar, fussing around it to fend off predators. Harvard University Professor Naomi Pierce was one of the first to identify that the food caterpillars provide for ants serves as an important currency of exchange in these interactions.

The larvae of some species of butterfly, such as the Bathurst Copper, are even shepherded by ants up trees to feed among the branches, then taken back to the ants' own nest to shelter during the day. Others even prey on the ants' own larvae, at the same time enjoying the hospitality of their nests.

Fact file:

When: All year round (depending on location)

Where: Lycaenids are found across most of Australia. Those in the tropics breed all year round, while species in temperate climates have only one or two generations per year, with the winter months passed in the egg, larval or pupal stage. Being cold blooded, temperature is very important to butterflies and some species will land even if the sun just goes behind a cloud for a minute.

Other info: How to find butterflies - After hatching, lycenid butterfly larvae look like slaters. They may produce an audible clicking sound if disturbed. Adults butterflies perch with wings closed, looking down and fly rapidly, sometime around tree tops, or may settle close to the ground. Because they fly in sunshine, you'll see occasional brilliant flashes of colour. Adults may have iridescent green, blue, purple, bronze or orange wings. Males can often be spotted as a silhouette against the sky, perched on a bare branch at the top of a tree. If a tree has swarms or clusters of ants, they may be tending larvae or pupae which are many times their size.

Imperial Blue

The beautiful Imperial Blue (Jalmenus evagoras) is another butterfly whose life is inextricably bound up with ants. Found along the east coast of Australia from Melbourne to Gladstone, the butterfly lays eggs which lie dormant during winter, then hatching in spring into brown and black caterpillars. The caterpillars can frequently be seen around Canberra, gathered together into football size masses clinging to the branches of wattle trees, attended by a thick covering of small black ants.

The larvae are studded with tiny glands which secrete nectar. On their backs they also have a nectary organ which ants use to 'milk' the caterpillar. The ants palpitate this organ, making it pucker and push up to squeeze out a fresh drop of nectar.

The larvae also attract and cultivate the ant's attention by 'talking' to them. Although they don't have a voice-box, they do have 'teeth' in the abdomen which are scraped against a series of ridges to produce grunts, hisses and a drumming sound.

Different calls are used in different situations: some are done spontaneously, others when the caterpillar is disturbed or when ants are nearby. Researchers have done experiments to find out how important this calling is. They 'silenced' butterfly pupae by gluing up the noise-making organs with shellac (a clear liquid-like varnish). It turned out that ants took longer to find pupae which had been silenced, and found them much less attractive than their singing counterparts, suggesting that calling is very important to their survival.

Enemy-free space

But just how important are the ants to the survival of the caterpillars? Researchers tested this by excluding ants from a tree where caterpillars were feeding, by placing a collar around the trunk. They compared the survival rate of caterpillars on trees where ants were present, and where they were absent, and found that caterpillars attended by ants had a 500 times greater survival rate than those left to fend for themselves. They also found much more evidence of caterpillar predation on ant-free trees such as sucked out skins and caterpillar carcasses wrapped in silk. It seems the ants create an enemy-free space, by fending off potential caterpillar predators, such as wasps, other ants, bugs, spiders and parasitoids.

However, life with ants does have a downside. Some butterfly predators actually use ants as cues to find caterpillars. Orb-weaving spiders, for example, which eat caterpillar and adult butterflies, prefer trees where ants are present.

Caterpillar cuckoos

The relationship between ant and butterfly is tenuous at best, and sometimes the evolutionary pendulum swings more towards one than the other. In the case of Illidge's ant-blue (Acrodipsas illidgei), a rare butterfly found only in small pockets of mature Queensland mangrove, the butterfly caterpillar eats the larvae of its ant host.

After the butterfly eggs hatch in early summer, ants pick the larvae up and lug them to their nests within the tree branches. Ants feed off the larval nectar secretions as it languishes in the ant nest, but at the same time, the butterfly larvae eat the ant's own larvae, sucking out every last drop of juice. Yet the ants continue to attend the butterfly larvae as they grow to an enormous size within the nest, running all over their bodies to lap up the caterpillar nectar.

How the caterpillar gets away with this outrageous behaviour is still something of a mystery. Lycaenid expert Professor Roger Kitching from Griffith University says the butterfly larvae use a chemical crypsis to disguise its true identify and fool the ant into letting it into nest. Ant-blues can produce substances which mimic the pheromones that ants use to communicate with each-other. Cloaked with pheromones, the butterfly caterpillar appear to the ant as one of its own kind.

However, once the butterfly pupae hatches into an adult, it loses this cloaking ability and must get out of the nest as quickly as possible or die. Newly emerged butterflies take a while to warm up and fly, and are very vulnerable. But the butterfly has another adaptation to help it - covering its newly emerged body are deciduous scales, which slough off if the insect is grabbed by an ant, enabling it to beat a hasty retreat.

Few colonies of ant-blue butterflies have been found, and the species was listed as endangered by the IUCN in 2000. However, a recent report from Environment Australia (Action Plan for Australian Butterflies 2003) suggests the butterfly's apparent rarity could be due to difficulty in detection, because it settles on high branches and rarely flies. However, while there is extensive suitable habitat remaining, a major threat to the butterfly's survival comes from spraying mangroves to control mosquitoes using toxic substances such as malathion.

Bye bye butterfly?

More and more people are noticing that there aren't as many butterflies around as there used to be. Land clearing, loss of habitat and use of pesticides must take the largest proportion of the blame. Butterflies which live in association with ants such as the Lycaenids are particularly vulnerable, because of the complexity of their life history.

The Federal Government's Action Plan for Butterflies 2003 recommends that 26 of Australia's 650 butterfly species be listed as threatened under the Environment Protection and Conservation Act - only one species is currently protected.

Most of the threats are from habitat loss, in particular, mangrove communities in NSW and QLD, coastal grasslands and paperbark forests, saline sedgelands in SA and TAS, dunes and healthlands in WA, summits of hilltops (because male butterflies often gather here), native grasslands and healthlands in SE Australia, old growth brigalow and bulloak forest in eastern Australia, mistletoes on trees in SA, lowland rainforest in eastern Australia, watercourse plant communities in eastern Australia, and native plant communities on islands.

While the beauty of butterflies makes them vulnerable to over-collection, it has also helped raise public interest in their protection. Now there are groups active in every state towards the conservation of butterflies. Charismatic butterflies, such as the Richmond Birdwing and the Eltham Copper, have helped communities raise the alarm and galvanise support in the past few years. In doing so, they have created a broader awareness of the plight of butterflies in Australia.